62. What is polymorphism?
This is very famous question and every interviewer asks this. Few people say polymorphism means multiple forms and they start giving example of draw function which is right to some extent but interviewer is looking for more detailed answer. Ability of base class pointer to call function from derived class at runtime is called polymorphism. For example, there is super class human and there are two subclasses software engineer and hardware engineer. Now super class human can hold reference to any of subclass because software engineer is kind of human. Suppose there is speak function in super class and every subclass has also speak function. So at runtime, super class reference is pointing to whatever subclass, speak function will be called of that class. I hope I am able to make you understand.

63. When to use NSMutableArray and when to use NSArray?
Normally we use mutable version of array where data in the array will change. For example, you are passing a array to function and that function will add some elements to that array or will remove some elements from array, then you will select NSMutableArray. When you don’t want to change you data, then you store it into NSArray. For example, the country names you will put into NSArray so that no one can accidentally modify it.

64. How is the app delegate is declared by Xcode project templates?
App delegate is declared as a subclass of UIResponder by Xcode project templates.

65. What is the purpose of UIWindow object?
The presentation of one or more views on a screen is coordinated by UIWindow object.

66. Whats the difference between frame and bounds?
The frame of a view is the rectangle, expressed as a location (x,y) and size (width,height) relative to the superview it is contained within. The bounds of a view is the rectangle, expressed as a location (x,y) and size (width,height) relative to its own coordinate system (0,0).

67. What is @interface?
It’s a keyword used to declare the Class.

68. What is @implementation?
It’s a keyword used to define the Class.

69. Garbage collector in iPhone?
iOS has got the ARC ( Automated reference counting ). Objective C does not have a garbage collector rather it uses the reference counting algorithm to manage the memory. This was the developers task until Apple launched iOS 5.0. Again if you are targeting iOS 4.0 or earlier , ARC is no more a choice for you.

70. What is delegate?
Delegate is an object that handles the events happening on an object. To do that delegate has to follow a protocol specifying the task it is going to handle.